Viral and Bacterial Epibionts in Thermally-Stressed Corals

Bioactivity responses to changes in mucus-associated bacterial composition between healthy and bleached Porites lobata corals

This study aims to investigate how bioactivities of the coral surface mucus layer (SML) respond to changes in mucus-associated bacterial communities between bleached and healthy Porites lobata corals in Nha Trang Bay, Vietnam. The findings suggested that significant shifts in the mucus-associated bacterial communities were related to changes in coral health states from bleached to healthy P. lobata colonies (p < 0.05), while bacterial compositions were not significantly different across seasons and locations (p > 0.05). Of which 8 genera, Shewanella, Fusibacter, Halodesulfovibrio, Marinifilum, Endozoicomonas, Litoribacillus, Algicola, and Vibrio were present only in the SML of bleached coral while absent in the SML of the healthy one. As compared with the bleached SML, the healthy SML demonstrated stronger antibacterial activity against a coral bleaching pathogen, V. coralliilyticus, higher antitumor activity against HCT116 cell accompanied with increased induction of cleaved PARP and accelerated cell nucleic apoptosis and cycle arrest at S and G2/M phases exhibiting several typical characteristics, cell shrinkage, lost cell contact, and apoptotic body formation. Moreover, putative compounds detected at 280 nm in the healthy SML were obviously higher than those in the bleached one, probably they could be bioactive molecules responsible for competitively exclusion of pathogens, Algicola and Vibrio, from the healthy SML.

Pre-Bleaching Coral Microbiome Is Enriched in Beneficial Taxa and Functions

Coral reef health is tightly connected to the coral holobiont, which is the association between the coral animal and a diverse microbiome functioning as a unit. The coral holobiont depends on key services such as nitrogen and sulfur cycling mediated by the associated bacteria. However, these microbial services may be impaired in response to environmental changes, such as thermal stress. A perturbed microbiome may lead to coral bleaching and disease outbreaks, which have caused an unprecedented loss in coral cover worldwide, particularly correlated to a warming ocean. The response mechanisms of the coral holobiont under high temperatures are not completely understood, but the associated microbial community is a potential source of acquired heat-tolerance. Here we investigate the effects of increased temperature on the taxonomic and functional profiles of coral surface mucous layer (SML) microbiomes in relationship to coral-algal physiology. We used shotgun metagenomics in an experimental setting to understand the dynamics of microbial taxa and genes in the SML microbiome of the coral Pseudodiploria strigosa under heat treatment. The metagenomes of corals exposed to heat showed high similarity at the level of bacterial genera and functional genes related to nitrogen and sulfur metabolism and stress response. The coral SML microbiome responded to heat with an increase in the relative abundance of taxa with probiotic potential, and functional genes for nitrogen and sulfur acquisition. Coral-algal physiology significantly explained the variation in the microbiome at taxonomic and functional levels. These consistent and specific microbial taxa and gene functions that significantly increased in proportional abundance in corals exposed to heat are potentially beneficial to coral health and thermal resistance.

Physical and cellular impact of environmentally relevant microplastic exposure on thermally challenged Pocillopora damicornis (Cnidaria, Scleractinia)

Microplastic pollution is an increasing threat to coral reefs, which are already strongly challenged by climate change-related heat stress. Although it is known that scleractinian corals can ingest microplastic, little is known about their egestion and how microplastic exposure may impair corals at physiological and cellular levels. In addition, the effects of microplastic pollution at current environmental concentration have been little investigated to date, particularly in corals already impacted by heat stress. In this study, the combined effects of these environmental threats on Pocillopora damicornis were investigated from a physical and cellular perspective. Colonies were exposed to three concentrations of polyethylene microplastic beads (no microplastic beads: [No MP], 1 mg/L: [Low MP]; 10 mg/L: [High MP]), and two different temperatures (25 °C and 30 °C) for 72 h. No visual signs of stress in corals, such as abnormal mucus production and polyp extroflection, were recorded. At [Low MP], beads adhered to colonies were ingested but were also egested. Moreover, thermally stressed colonies showed a lower adhesion and higher egestion of microplastic beads. Coral bleaching was observed with an increase in temperature and microplastic bead concentration, as indicated by a general decrease in chlorophyll concentration and Symbiodiniaceae density. An increase in lipid peroxidation was measured in colonies exposed to [Low MP] and [High MP] and an up-regulation of stress response gene hsp70 was observed due to the synergistic interaction of both stressors. Overall, our findings showed that heat stress still represents the main threat to P. damicornis, while the effect of microplastics on coral health and physiology may be minor, especially at control temperature. However, microplastics could exacerbate the effect of thermal stress on cellular homeostasis, even at [Low MP]. While reducing ocean warming is critical for preserving coral reefs, effective management of emerging threats like microplastic pollution is equally essential.

Increased cyanophage infection at the bottom of the euphotic zone, especially in the fall

Picocyanobacteria contribute greatly to offshore primary production with cells extending through the deep euphotic zone. Literature indicates high viral infection of cyanobacteria in ocean transition zones. We postulate that the bottom of the euphotic zone is a transition zone, where communities transition from phototrophic to aphotic processes. We use single-copy core genes to examine cyanophage to cyanobacteria ratios in cellular metagenomes in the subtropical North Atlantic and Pacific. Cyanophage to cyanobacteria terL/rpoB ratios generally increase to >10 in the deep euphotic zone. As light levels decrease in the fall, Prochlorococcus in the deep euphotic zone experience reduced light levels. We find clear differences between spring (Geotraces GA02) and fall (GA03) in the North Atlantic, with terL/rpoB ratios increasing to >40 in the fall. When examining 23 months of the North Pacific Hawaii Ocean Timeseries, the depth of elevated cyanophage to cyanobacteria ratios in cellular metagenomes negatively correlated with surface photosynthetic radiation (PAR), particularly with the change in PAR, which reflected the season. In fall, all picocyanobacteria ecotypes were found at depths enriched with viruses, while in summer, only low light ecotypes were affected. Thus, we find high cyanophage infection both in the deep euphotic zone and during seasonal transitions.

Thermal stress triggers productive viral infection of a key coral reef symbiont

Climate change-driven ocean warming is increasing the frequency and severity of bleaching events, in which corals appear whitened after losing their dinoflagellate endosymbionts (family Symbiodiniaceae). Viral infections of Symbiodiniaceae may contribute to some bleaching signs, but little empirical evidence exists to support this hypothesis. We present the first temporal analysis of a lineage of Symbiodiniaceae-infecting positive-sense single-stranded RNA viruses ("dinoRNAVs") in coral colonies, which were exposed to a 5-day heat treatment (+2.1 °C). A total of 124 dinoRNAV major capsid protein gene "aminotypes" (unique amino acid sequences) were detected from five colonies of two closely related Pocillopora-Cladocopium (coral-symbiont) combinations in the experiment; most dinoRNAV aminotypes were shared between the two coral-symbiont combinations (64%) and among multiple colonies (82%). Throughout the experiment, seventeen dinoRNAV aminotypes were found only in heat-treated fragments, and 22 aminotypes were detected at higher relative abundances in heat-treated fragments. DinoRNAVs in fragments of some colonies exhibited higher alpha diversity and dispersion under heat stress. Together, these findings provide the first empirical evidence that exposure to high temperatures triggers some dinoRNAVs to switch from a persistent to a productive infection mode within heat-stressed corals. Over extended time frames, we hypothesize that cumulative dinoRNAV production in the Pocillopora-Cladocopium system could affect colony symbiotic status, for example, by decreasing Symbiodiniaceae densities within corals. This study sets the stage for reef-scale investigations of dinoRNAV dynamics during bleaching events.

Microbial community and transcriptional responses to increased temperatures in coral Pocillopora damicornis holobiont

A few studies have holistically examined successive changes in coral holobionts in response to increased temperatures. Here, responses of the coral host Pocillopora damicornis, its Symbiodiniaceae symbionts, and associated bacteria to increased water temperatures were investigated. High temperatures induced bleaching, but no coral mortality was observed. Transcriptome analyses showed that P. damicornis responded more quickly to elevated temperatures than its algal symbionts. Numerous genes putatively associated with apoptosis, exocytosis, and autophagy were upregulated in P. damicornis, suggesting that Symbiodiniaceae can be eliminated or expelled through these mechanisms when P. damicornis experiences heat stress. Furthermore, apoptosis in P. damicornis is presumably induced through tumour necrosis factor and p53 signalling and caspase pathways. The relative abundances of several coral disease-associated bacteria increased at 32°C, which may affect immune responses in heat-stressed corals and potentially accelerates the loss of algal symbionts. Additionally, consistency of Symbiodiniaceae community structures under heat stress suggests non-selective loss of Symbiodiniaceae. We propose that heat stress elicits interrelated response mechanisms in all parts of the coral holobiont.

Coral Reef Microorganisms in a Changing Climate

Coral reefs are one of the most diverse and productive ecosystems on the planet, yet they have suffered tremendous losses due to anthropogenic disturbances and are predicted to be one of the most adversely affected habitats under future climate change conditions. Coral reefs can be viewed as microbially driven ecosystems that rely on the efficient capture, retention, and recycling of nutrients in order to thrive in oligotrophic waters. Microorganisms play vital roles in maintaining holobiont health and ecosystem resilience under environmental stress; however, they are also key players in positive feedback loops that intensify coral reef decline, with cascading effects on biogeochemical cycles and marine food webs. There is an urgent need to develop a fundamental understanding of the complex microbial interactions within coral reefs and their role in ecosystem acclimatization, and it is important to include microorganisms in reef conservation in order to secure a future for these unique environments.

Coral metabolite gradients affect microbial community structures and act as a disease cue

Corals are threatened worldwide due to prevalence of disease and bleaching. Recent studies suggest the ability of corals to resist disease is dependent on maintaining healthy microbiomes that span coral tissues and surfaces, the holobiont. Although our understanding of the role endosymbiotic microbes play in coral health has advanced, the role surface-associated microbes and their chemical signatures play in coral health is limited. Using minimally invasive water sampling, we show that the corals Acropora and Platygyra harbor unique bacteria and metabolites at their surface, distinctly different from surrounding seawater. The surface metabolites released by the holobiont create concentration gradients at 0–5 cm away from the coral surface. These molecules are identified as chemo-attractants, antibacterials, and infochemicals, suggesting they may structure coral surface-associated microbes. Further, we detect surface-associated metabolites characteristic of healthy or white syndrome disease infected corals, a finding which may aid in describing effects of diseases.

Michael Ochsenkühn et al. look at the microbial and metabolic composition of coral surfaces and the surrounding seawater. They find that the metabolites found on the surface of the coral create a concentration gradient that influences the surrounding microbiome.

Impact of inorganic UV filters contained in sunscreen products on tropical stony corals (Acropora spp.)

Most coral reefs worldwide are threatened by natural and anthropogenic impacts. Among them, the release in seawater of sunscreen products commonly used by tourists to protect their skin against the harmful effects of UV radiations, can affect tropical corals causing extensive and rapid bleaching. The use of inorganic (mineral) filters, such as zinc and titanium dioxide (ZnO and TiO₂) is increasing due to their broad UV protection spectrum and their limited penetration into the skin. In the present study, we evaluated through laboratory experiments, the impact on the corals Acropora spp. of uncoated ZnO nanoparticles and two modified forms of TiO₂ (Eusolex®T2000 and Optisol™), largely utilized in commercial sunscreens together with organic filters. Our results demonstrate that uncoated ZnO induces a severe and fast coral bleaching due to the alteration of the symbiosis between coral and zooxanthellae. ZnO also directly affects symbiotic dinoflagellates and stimulates microbial enrichment in the seawater surrounding the corals. Conversely, Eusolex® T2000 and Optisol™ caused minimal alterations in the symbiotic interactions and did not cause bleaching, resulting more eco-compatible than ZnO. Due to the vulnerability of coral reefs to anthropogenic impacts and global change, our findings underline the need to accurately evaluate the effect of commercial filters on stony corals to minimize or avoid this additional source of impact to the life and resilience ability of coral reefs.

Association of coral algal symbionts with a diverse viral community responsive to heat shock

BACKGROUND

Stony corals provide the structural foundation of coral reef ecosystems and are termed holobionts given they engage in symbioses, in particular with photosynthetic dinoflagellates of the genus Symbiodinium. Besides Symbiodinium, corals also engage with bacteria affecting metabolism, immunity, and resilience of the coral holobiont, but the role of associated viruses is largely unknown. In this regard, the increase of studies using RNA sequencing (RNA-Seq) to assess gene expression provides an opportunity to elucidate viral signatures encompassed within the data via careful delineation of sequence reads and their source of origin.

RESULTS

Here, we re-analyzed an RNA-Seq dataset from a cultured coral symbiont (Symbiodinium microadriaticum, Clade A1) across four experimental treatments (control, cold shock, heat shock, dark shock) to characterize associated viral diversity, abundance, and gene expression. Our approach comprised the filtering and removal of host sequence reads, subsequent phylogenetic assignment of sequence reads of putative viral origin, and the assembly and analysis of differentially expressed viral genes. About 15.46% (123 million) of all sequence reads were non-host-related, of which <1% could be classified as archaea, bacteria, or virus. Of these, 18.78% were annotated as virus and comprised a diverse community consistent across experimental treatments. Further, non-host related sequence reads assembled into 56,064 contigs, including 4856 contigs of putative viral origin that featured 43 differentially expressed genes during heat shock. The differentially expressed genes included viral kinases, ubiquitin, and ankyrin repeat proteins (amongst others), which are suggested to help the virus proliferate and inhibit the algal host's antiviral response.

CONCLUSION

Our results suggest that a diverse viral community is associated with coral algal endosymbionts of the genus Symbiodinium, which prompts further research on their ecological role in coral health and resilience.